GCaMP3 is a genetically encoded fluorescent Ca[2+] indicator, derived by mutagenesis of GCaMP2. GCaMP3 is a fusion protein composed of an N-terminal residue RSET leader sequence (which includes Tag:polyHis), the calmodulin-binding peptide M13, a circularly permuted fluorescent protein and calmodulin. The circularly permuted fluorescent protein is interrupted at residue 145, and is based on EGFP, with the mutations M153K, T203V and N60D in addition to those present in GCaMP2. GCaMP3 also contains a deletion of residue R2 relative to GCaMP2 (FBrf0209330). For detailed kinetic and spectral properties, see FBrf0209330. For a comparison of GCaMP3 with other GCaMP series sensors, see FBrf0219564.